A releasing hormone or releasing factor is a hormone whose sole purpose is to control the release of another hormone. These are also called hypophysiotropic or hypothalamic hormones. The use of the term factor was employed for some time pending the establishment of the molecular structure of the hormones. When this had been fully established the hormones were referred to as releasing hormones.[1] The main releasing hormones secreted by the hypothalamus are:

For example, TRH is released from the hypothalamus in response to low levels of secretion of thyroid stimulating hormone (TSH) from the pituitary gland. The TSH in turn is under feedback control by the thyroid hormones T4 and T3. When the levels of TSH are too high, they feed back on the brain to shut down the secretion of TRH. Synthetic TRH is also used by physicians as a test of TSH reserve in the pituitary gland as it should stimulate the release of TSH and prolactin from this gland.

Releasing hormones are sometimes known as liberins. For example, TRH may be known as thyroliberin. Inhibiting hormones may be known as statins.[citation needed] For example, dopamine (which inhibits prolactin release) may be called prolactostatin.

Roger Guillemin and Andrew W. Schally were awarded the Nobel Prize in Physiology and Medicine in 1977 for their contributions to understanding "the peptide hormone production of the brain"; these scientists independently first isolated TRH and GnRH and then identified their structures.[4]